With the progress of the power electronic devices and the control technology, the flexible DC capacity and voltage level are getting higher and higher. Multi-terminal flexible DC transmission is more advantageous in operation flexibility and reliability than two-terminal flexible DC transmission, and moreover, building the multi-terminal flexible DC transmission can meet the requirements of grid interconnection, power supply of urban power grids, and interconnection of multiple wind power plants (new energy resources), which has a great significance. However, the multi-terminal flexible DC transmission system is more complex and higher in operational requirements than the two-terminal flexible DC transmission. It is a focus in industry as regards how to keep the multi-terminal flexible DC transmission system to operate safely and reliably, and in particular, there has not yet been a good solution for connecting a non-operating station to an operating system in a multi-terminal flexible DC transmission system. At present, to connecting a non-operating station to an operating system, the operating system is firstly out of operation, and then the entire multi-terminal flexible DC transmission system is restarted. This method has the following disadvantages:
(1) the outage probability of the multi-terminal flexible DC transmission system is increased;
(2) if the operating system is connected to an important load, a significant loss will be caused;
(3) if the operating system is connected to a new energy source, such as a wind power plant, the new energy resource is caused to be out of operation and put into operation again, thus reducing the energy utilization rate.
In order to improve the stability and reliability of the multi-terminal flexible DC transmission and give full play to the advantages of the multi-terminal flexible DC transmission, there is a need for a more effective control method for incorporating a non-operating station into an operating system, and the present application is developed for this purpose.